This invention relates to a connection between mechanical parts that is locked so it does not inadvertently uncouple.
On a workover rig used to recomplete or do remedial work on oil or gas wells, a power swivel is often used to rotate a tubing string, make up or break the threaded connections between adjacent tubing joints and the like. A safety valve is typically provided immediately below the power swivel to control blow out pressure coming up the tubing string. Typical safety valves include housing sections that are threaded together, a valve mechanism inside the housing and an operator recessed into the side of the housing that can be manipulated with a suitable tool. Because the power swivel provides the torque to make up and break the threaded connections between tubing joints, the safety valve must transmit both clockwise and counterclockwise torque. Safety valves are typically made so that the clockwise torque, viewed from above, used to tighten tubing joints also tightens the threaded housing sections of the safety valve. This means that counterclockwise torque, used to break the tubing joints, also tends to unthread the safety valve housing. Thus, a torque which unscrews the housing is applied periodically to the safety valve and safety valve housings are occasionally unintentionally uncoupled.
The prior art solution to this problem has been to provide one or more set screws extending through one of the housing sections into the other. When it is desired to uncouple the housing sections to replace the valve seats, for example, the set screws are removed and the housing sections unthreaded. One often finds the set screws are cracked or broken, the housing adjacent the set screw passages are cracked or the threads of the set screw passages to are deformed making it extremely difficult to remove the set screws. This often requires redrilling a new series of set screw passages or replacing the housing. Breaking of the set screws, cracking of the housing or deformation of the set screw passages is clear evidence of the forces applied to the housing by the power swivel.
Disclosures of some interest relative to this invention are found in U.S. Pat. Nos. 48,667; 132,755; 387,475; 473,818; 754,741; 1,407,335 and 5,348,349.
In this invention, a locked connection between mechanical parts is provided to prevent inadvertent uncoupling movement between the parts. This is accomplished by providing an outwardly facing circular groove on an interior part and one or more passages on an exterior part tangentially intersecting the groove. The passage accordingly provides an intermediate open section aligned with the circular groove in a connected position of the mechanical parts. The intermediate passage section and the groove cooperate to provide a passageway of a preferred cross-section, usually circular.
A locking pin extends through the passage and groove preventing uncoupling movement of the mechanical parts. Suitable means, such as a set screw or shear pin, prevents axial movement of the locking pin out of the passage. In this manner, the large force tending to uncouple the mechanical parts is taken by the locking pin. The set screw or shear pin has to resist a much smaller force tending to move the locking pin out of the passage.
When uncoupling forces are applied to the connection, part of the locking pin is placed in shear, specifically a path corresponding to the intersection of the circular groove and the locking pin. The long dimension of this intersection is curved because the groove is curved. The maximum short dimension of this intersection is the cross-section of the locking pin. With this geometry, the available area of the locking pin that is in shear is much larger than the diameter of the locking pin, usually by a factor of at least two and one half, preferably of at least three and ideally at least four.
In one embodiment of this invention, the locked connection prevents unthreading movement of a threaded connection, such as in a tubing safety valve. In another embodiment of this invention, the locked connection prevents axial uncoupling movement of adjacent unthreaded components, such as an abutment used to position a valve cartridge in an operative position. A substantial advantage of this invention is that the passages in the box end are always aligned with the groove in the pin end in the made up position of the threaded components. This is in contrast to a situation where the passage in the pin end extends through the body of the pin end so the pin passage-and box passage must be rotationally aligned.
It is an object of this invention to provide an improved locked connection between adjacent mechanical parts which are capable of uncoupling movement.
Another object of this invention is to provide an improved locked connection between mechanical parts which are threaded together.
A further object of this invention to provide an improved locked connection between valve housing sections.
Another object of this invention is to provide an improved locked connection in which the main loads are taken by a locking pin in which the geometry of the assembly provides a large area in shear.
A further object of this invention is to provide an improved locked connection in which a long dimension of a locking pin is placed in shear.
These and other objects and advantages of this invention will become more fully apparent as this description proceeds, reference being made to the accompanying drawings and appended claims.